The present invention relates to a transceiver and a method of tuning a transceiver. More particularly, the present invention relates to a device and method for tuning a transceiver at the modulation section while the local oscillator at the frequency conversion section remains fixed.
Many modern communication systems include antennas with transmitter/receiver devices (transceivers) for simultaneously transmitting and receiving independent radio frequency (RF) signals. When the RF signal being transmitted or received has a carrier signal with a high frequency, i.e., greater than 1 GHz, it is often particularly desirable for the transceiver to be a physically located close to the feed of the antenna. High frequency RF signals may lose considerable signal strength and/or suffer interference when they are carried from the antenna to a remote transceiver. Hence, a physically proximity between the antenna and the transceiver is often desirable.
Generally, in high frequency transceivers, the data being transmitted (which may be voice, digital data, etc.) is baseband data which is modulated up to the carrier frequency in a two or three stage process. In the first one or two stages, the baseband data to be transmitted is modulated to an intermediate frequency ("IF"); then, in the last stage, the IF signal is up converted to an RF signal which may be amplified and applied to the antenna (through a conventional antenna coupling) for broadcast. In the case of a received signal, the sequence of steps is reversed.
In most communication systems, it is desirable to be able to use a single transceiver for the transmission/receipt of differing RF signals having a range of carrier frequencies. Thus, the transceiver is able to communicate on a number of "channels" in the RF spectrum. Indeed, in some applications, e.g., in frequency hopping systems, it is desirable that a single communication utilize on a succession of carriers having different center frequencies. In order to be able to "tune" the transceiver to a range of carrier frequencies, it is known to use the modulator section of the transceiver to modulate the baseband data to a predetermined IF frequency and to vary the frequency of the up converter (or down converter) to produce the range of desired carrier frequencies. Generally, the circuits needed to perform a variable (or selective) conversion are considerably more complex and contain significantly more components than do circuits which are capable of up converting by a single frequency. For example, components (e.g., filters, amplifiers, etc.) used in such circuits may require additional bandwidth capability and must operate over a range of conditions. Moreover, circuits which operate at a very high frequency (such as 1 GHZ) tend to be much more expensive, more difficult to maintain, and more susceptible to external interference than circuits which operate at IF frequencies. Accordingly, in terms of general cost and complexity, communication systems in which the complex circuits operate in the IF range are desirable.
Some prior art transceivers have one conversion section for down-converting a received RF signal to an IF signal and for up-converting an IF signal to the RF regime. Transceivers having only one conversion section may have a separate local oscillator for the transmit path and a separate local oscillator for the receive path or they may have a single local oscillator serving both the transmit and the receive paths.
The transceiver apparatus disclosed in Yamamoto U.S. Pat. No. 4,907,291 include a transceiver unit having a conversion section and a separate main unit having a modulation section. The transceiver unit is located at the feed of the antenna and the main unit is located some distance away from the transceiver unit. The transceiver unit and the main unit are separate devices connected by a single RF cable and therefore each unit requires a multiplexing scheme for transmitting and receiving their respective signals. Moreover, the signal transmitted between the main and transceiver units is at a very high frequency, exceeding 1 GHz. A problem is that because the conversion stage is located in the main unit and the modulation stage is located in the transceiver unit, relatively high frequency signals are passed between the units thereby increasing the signal attenuation due to cable loss and the complexity of the equipment needed to handle the signals.
Yet a further problem is that the increased signal attenuation must be compensated with higher gain amplifiers which complicates the overall design of the system and increases cost.
It is accordingly an object of the present invention to provide a novel method and apparatus for transmitting and receiving independent RF signals having a range of carrier frequencies wherein frequency conversion is performed at relatively low frequency levels.
It is a further object of the present invention to provide a novel method and apparatus for transceiving RF signals which has a relatively low cost due to reduced component counts and circuit complexity.
It is yet a further object of the present invention to provide a novel method and apparatus for transceiving RF signals without complicated frequency multiplexers.
It is still another object of the present invention to provide a novel method and apparatus for transceiving RF signals where the signal received and the signal transmitted are remotely controlled.
It is still another object of the present invention to provide a novel method and apparatus for transceiving RF signals in which the signals passed through cables are kept relatively low in frequency.